Enormous activity has taken place, and resources expended in recent years towards defining a broad range of future satellite communication (satcom) system architectures that are striving to provide global services. These services accommodate both stationary and mobile users, and range in sophistication from one-way paging and messaging to two-way high-quality voice. During recent years, activities have also included many filings with the FCC in order to receive approval to proceed with development, launch, and operations by the mid-to-late 1990's. the concepts proposed have been quite diverse and encompass:
1. Low Earth Orbits (LEO)--e.g., Motorola's Iridium, Loral's Globalstar, and Orbital Sciences Orbcomm--which propose operational constellations with satellite quantities ranging from 26 to 66.
2. Medium Earth Orbits--e.g., TRW's Odyssey--which propose operational constellations with satellite quantities on the order of 12.
3. Geosynchronous Orbits (GEO)--e.g, American Mobile Satellite in geostationary orbit--which either provide regional coverage, or would require on the order of 4 to 5 satellites for global coverage at latitudes up to 70.degree..
Many tradeoffs have been addressed in the literature describing these systems. Examples include:
Benefits of decreasing orbital altitude:
1. Per satellite launch cost decreases.
2. User and satellite transmit power and/or antenna complexity decrease.
3. Propagation delay decreases.
Benefits of increasing orbital altitude:
1. Quantity of operational satellites and supporting ground stations decrease.
2. Satellite handover complexity decreases, and is eliminated in the GEO architecture.
3. For the GEO architecture, full operational capability can evolve--one geographic region at a time--per launch.
4. Satellite life increases for altitudes above the Van Allen Belts.
Other considerations:
1. A non-stationary satellite system is more amenable to providing attractive user-to-satellite elevation angles.
2. The quantity of operational satellites is heavily driven by truly global coverage (100% of the time) vs. alternatives (e.g., only land masses or less than 100% at extreme latitudes), and the minimum acceptable user-to-satellite elevational angle.
Ultimately, the drivers for a successful commercial system must reduce to the combination of cost, service and quality benefits, and reliability. While many detailed assumptions enter into cost calculations, FIG. 1 presents the results of cost trades analysis (done by others), for a specific mobile satcom system application supporting two-way voice, that encompass satellite complexity, launch cost and satellite quantity. These results demonstrate the cost effectiveness of Odyssey MEO system. At the same time, however, these results provide the key message that GEO system cost is not much greater--even though the individual GEO satellite cost may be high--given its much smaller constellation size, fewer ground stations, simpler control, and the longer lifetime of each satellite.
The insight gained from the analysis shown in FIG. 1 are actually much more profound since they provide a much broader and exciting message concerning GEO systems:
If sufficient implementation (weight, size, power, complexity) and launch cost reduction per GEO satellite can be achieved, while simultaneously enhancing coverage and satellite/user elevation angles, then a GEO constellation potentially emerges as the lowest cost alternative for providing global, mobile satellite communications. In addition, the GEO cost benefit can be maximized by targeting a carefully selected set of mobile and non-mobile applications that are ideally suited for GEO satellites.
The object of this invention is to provide an improved satellite communication system using geosynchronous.